Many species of Thyme have been widely used in Moroccan folk medicine as anti-inflammatory, antioxidant and antinociceptive agents. This study was designed to examine the chemical composition and the in vitro antitumor activity of the essential oils and various extracts of thyme species collected in different regions of Morocco. The essential oil, obtained by hydrodistillation, and the various extracts, obtained by Soxhlet extraction, using solvents of varying polarity, were analysed by gas chromatography coupled to mass spectrometry (GC-MS). Both major and trace components were analysed. Overall, the major constituents in the chemical composition of Moroccan thyme populations were carvacrol, thymol, borneol and p-cymene. The rate of these components can hit respectively to 85%, 42%, 59%, and 23%. Furthermore, the essential oils as well as two pure products (carvacrol and thymol) were tested for their antitumoral activity against P815 mastocytoma cell line. While all these products showed a dose dependent cytotoxic effect, the carvacrol was the most cytotoxic one compared to the others. Interestingly, when these products were tested against the normal human peripheral blood mononuclear cells, they show a proliferative effect instead of a cytotoxic one.